Accounting for spectroscopic effects in laser-based open-path eddy covariance flux measurements.

A significant portion of the production and consumption of trace gases (e.g. CO2 , CH4 , N2 O, NH3 , etc.) by world ecosystems occurs in areas without sufficient infrastructure or easily available grid power to run traditional closed-path flux stations. Open-path analyzer design allows such measurements with power consumption 10-150 times below present closed-path technologies, helping to considerably expand the global coverage and improve the estimates of gas emissions and budgets, informing the remote sensing and modeling communities and policy decisions, all the way to IPCC reports. Broad-band nondispersive infrared devices have been used for open-path CO2 and H2 O measurements since the late 1970s, but since recently, a growing number of new narrow-band laser-based instruments are being rapidly developed. The new design comes with its own challenges, specifically: (a) mirror contamination, and (b) uncontrolled air temperature, pressure and humidity, affecting both the gas density and the laser spectroscopy of the measurements. While the contamination can be addressed via automated cleaning, and density effects can be addressed via the Webb-Pearman-Leuning approach, the spectroscopic effects of the in situ temperature, pressure and humidity fluctuations on laser-measured densities remain a standing methodological question. Here we propose a concept accounting for such effects in the same manner as Webb et al. proposed to account for respective density effects. Derivations are provided for a general case of flux of any gas, examined using a specific example of CH4 fluxes from a commercially available analyzer, and then tested using "zero-flux" experiment. The proposed approach helps reduce errors in open-path, enclosed, and temperature- or pressure-uncontrolled closed-path laser-based flux measurements due to the spectroscopic effects from few percents to multiple folds, leading to methodological advancement and geographical expansion of the use of such systems providing reliable and consistent results for process-level studies, remote sensing and Earth modeling applications, and GHG policy decision-making.

State-resolved thermalization of singlet and mixed singlet-triplet states of CH2.

The role of mixed states in the collision-induced thermalization, intersystem crossing, and reactive loss of CH(2) (~a (1)A1) has been monitored using Doppler-resolved transient frequency modulation absorption spectroscopy. Singlet CH(2) is produced in a hot initial distribution of translation and rotational energy states in the 308 nm photodissociation of ketene in a large excess of argon. Collisions with Ar and ketene cool the translational and rotational degrees of freedom, while depleting the total singlet CH(2) population through reaction and intersystem crossing. Direct monitoring of the time-dependent populations of rotational levels containing mixed singlet and triplet character reveals a rapid interconversion between the two components, but no discernable difference between the kinetics of the pure singlet and mixed states at longer times.

Correlated product distributions from ketene dissociation measured by dc sliced ion imaging.

Speed distributions of spectroscopically selected CO photoproducts of 308 nm ketene photodissociation have been measured by dc sliced ion imaging. Structured speed distributions are observed that match the clumps and gaps in the singlet CH2 rotational density of states. The effects of finite time gates in sliced ion imaging are important for the accurate treatment of quasicontinuous velocity distributions extending into the thickly sliced and fully projected regime, and an inversion algorithm has been implemented for the special case of isotropic fragmentation. With accurate velocity calibration and careful treatment of the velocity resolution, the new method allows us to characterize the coincident rotational state distribution of CH2 states as a smoothly varying deviation from an unbiased phase space theory (PST) limit, similar to a linear-surprisal analysis. High-energy rotational states of CH2 are underrepresented compared to PST in coincidence with all detected CO rotational states. There is no evidence for suppression of the fastest channels, as had been reported in two previous studies of this system by other techniques. The relative contributions of ground and first vibrationally excited singlet CH2 states in coincidence with selected rotational states of CO (upsilon=0) are well resolved and in remarkably good agreement with PST, despite large deviations from the PST rotational distributions in the CH2 fragments. At 308 nm, the singlet CH2 (upsilon2=0) and (upsilon2=1) channels are 2350 and 1000 cm(-1) above their respective thresholds. The observed vibrational branching is consistent with saturation at increasing energies of the energy-dependent suppression of rates with respect to the PST limit, attributed to a tightening variational transition state.

Observation of the c1A1 state of methylene by optical-optical double resonance.

We report the observation of the rotationally resolved spectrum of the c1A1 state of CH2 via sequential single-photon absorptions at visible and near-infrared wavelengths. Direct absorption from the lowest singlet state a1A1 to c1A1 occurs in the near UV, but it is weak because it corresponds to a two electron transition between the dominant single configuration approximations to the electronic wave functions. Some absorption lines in the c-a system were originally reported in 1966 [G. Herzberg and J. W. C. Johns, Proc. R. Soc. London, Ser. A 295, 107 (1966)], but the weak spectra could not be assigned at the time. Interest in the c1A(1) state was rekindled by recent ab initio results [S. N. Yurchenko, P. Jensen, Y. Li, R. J. Buenker, and P. R. Bunker, J. Mol. Spectrosc. 208, 136 (2001)] which prompted the present work. The new spectra provide accurate energies for rotational levels in the v2linear =11,l = 1 level of the state, and permit assignment of most of the line positions measured by Herzberg and Johns. The double-resonance technique used may be easily extended to the measurement of lower rovibrational levels in the electronic state and possibly also to access the d1A2 state which is theoretically expected to lie at similar energies but, for symmetry reasons, is not accessible from the lowest singlet state in a single electric-dipole transition.

Accurate ionization potentials for UO and UO2: a rigorous test of relativistic quantum chemistry calculations.

Accurate ionization potential (IP) measurements provide essential thermodynamic information and benchmark data that can be used to evaluate the validity of electronic structure models. Calculations of the first IP of UO2 using relativistic methods consistently predict values that are approximately 0.7 eV higher than the accepted experimental value. The present measurements validate the theoretical calculations and show that the previous determinations corresponded to the ionization of thermally excited molecules. Similarly, new measurements of the IP for UO show that the currently accepted value is too low by 0.4 eV.

Electronic spectroscopy and ionization potential of UO2 in the gas phase.

The electronic spectroscopy of UO(2) has been examined using multiphoton ionization with mass-selected detection of the UO(2) (+) ions. Supersonic jet cooling was used to reduce the spectral congestion. Twenty-two vibronic bands of neutral UO(2) were observed in the range from 17,400 to 32,000 cm(-1). These bands originated from the U(5fphi(u)7ssigma(g))O(2) X (3)Phi(2u) and (3)Phi(3u) states. The stronger band systems are attributed to metal-centered 7p<--7s transitions. Threshold ionization measurements were used to determine the ionization potentials of UO and UO(2). These were found to be higher than the values obtained previously from electron impact measurements but in agreement with the results of recent theoretical calculations.